Surface modification method for light metal casting

ABSTRACT

There is provided a surface modification method for a light metal casting that enables, with Friction Stir Processing, to further refine a surface at a portion at which the strength is especially required. A surface modification method for a light metal casting with Friction Stir Processing in which a rotating shaft and a rotator are rotated and fed while the rotating shaft and the rotator are being pressed against a surface of a casting to modify the surface of the casting, the method includes feeding the rotating shaft and the rotator while rotating the rotating shaft and the rotator in a manner such that a side at which a rotating direction of the rotating shaft and the rotator coincides with a feeding direction is positioned at a portion at which increase in the strength is desired with modification of the light metal casting.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2018-125783, filed on Jul. 2, 2018, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to a surface modification method for alight metal casting.

There is known a surface modification technique using Friction StirProcessing (FSP) for improving variations in properties of material andquality deterioration due to a coarsened solidification structure of alight metal casting, such as an aluminium alloy. Japanese UnexaminedPatent Application Publication No. 2004-255440 discloses, as a surfacemodification technique for a light metal casting using Friction StirProcessing, a technique for adding additives for modifying a metalstructure to the metal structure when the surface of a light metalcasting is refined with Friction Stir Processing.

SUMMARY

It is desirable that a member that can receive a large impact is brokenas designed when the member receives an impact. For example, membersused for a vehicle are desirably designed to be broken so that an impactreceived by a passenger is minimized when the vehicle receives animpact, such as a collision.

In a vehicle, members having complicated shapes are formed of lightmetal castings in many cases. For example, members in a vehicle, such asa suspension tower, a rear side member, and an engine cylinder block,are formed of light metal castings. Such members formed of light metalcastings are also desirably broken as designed when receiving an impact.

When a member formed of a light metal casting has a portion at whichstress tends to concentrate, the portion has the possibility ofincluding internal defects and having the lowest strength. For example,when a member formed of a light metal casting has a welded portion atwhich stress tends to concentrate, the possibility that a fractureoccurs at a region in the vicinity of the welded portion when an impactis applied is high. However, it is difficult, in terms of design, tocontrol occurrence of a fracture at a region in the vicinity of thewelded portion. Thus, when a member formed of a light metal casting hasa welded portion, it is required for a fracture to be caused not at aregion in the vicinity of the welded portion but at a portion at whichoccurrence of a fracture is easily controlled in design when an impactis applied. In other words, the strength at a region in the vicinity ofa welded portion needs to be increased to be higher than the strength ata portion at which occurrence of a fracture is easily controlled indesign. For this reason, in surface modification of a light metalcasting, it is desired to refine the metal structure at a portion atwhich the strength is especially required as compared to at otherportions.

In view of the above circumstance, a purpose of the present disclosureis to provide a surface modification method for a light metal castingthat enables, with Friction Stir Processing, to refine the metalstructure on the surface at a portion at which the strength isespecially required.

The present disclosure is a surface modification method for a lightmetal casting with Friction Stir Processing in which a rotating shaftand a rotator are rotated and fed while the rotating shaft and therotator are being pressed against a surface of a casting to modify thesurface of the casting, the method including feeding the rotating shaftand the rotator while rotating the rotating shaft and the rotator in amanner such that a side at which a rotating direction of the rotatingshaft and the rotator coincides with a feeding direction is positionedat a portion at which increase in the strength is desired withmodification of the light metal casting.

The inventor has found that, on a surface subjected to frictionstirring, the metal structure on a surface at a side (AS) at which therotating direction of a rotating shaft and a rotator coincides with thefeeding direction is refined as compared to at a side (RS) at which therotating direction of the rotating shaft and the rotator and the feedingdirection are opposite. By positioning a portion at which increase inthe strength is desired with modification of a light metal casting atthe side AS, it is possible to further refine the metal structure on thesurface at the portion at which increase in the strength is desired andto increase the strength after the surface modification.

In addition, the feeding in the same feeding direction may be performeda plurality of times, and each feeding may be performed by shifting, inparallel, a next feeding path from a previous feeding path by apredetermined width equal to or less than a diameter of the rotator. Byperforming feeding in this manner, a portion which has not beenpositioned at the side AS in the previous feeding path is to bepositioned at the side AS in the next feeding path or subsequent feedingpaths. Thus, it is possible to evenly refine the surface of a memberformed of a light metal casting.

Furthermore, the rotating shaft and the rotator may be fed according toa path circling along an edge of the light metal casting in a mannersuch that the side at which the rotating direction of the rotating shaftand the rotator coincides with the feeding direction is positioned atthe edge of the light metal casting. By positioning the side AS at theedge of the member formed of the light metal casting and the side RS atthe inner side, the metal structure on the surface at the portion in thevicinity of the edge is refined. Thus, it is possible to increase thestrength at the outer periphery of the member formed of the light metalcasting.

With the present disclosure, it is possible, with Friction StirProcessing, to further refine the surface at a portion at which thestrength is especially required.

The above and other objects, features and advantages of the presentdisclosure will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a configuration of a frictionstir apparatus used in a surface modification method for a light metalcasting according to the present embodiment;

FIG. 2 is a photomicrograph of the metal structure on the surface of amember formed of an aluminium alloy casting before modification;

FIG. 3 is a schematic diagram for explaining the outline of FrictionStir Processing of a friction stir apparatus;

FIG. 4 is a schematic diagram for explaining the difference in themodification effect by friction stirring;

FIG. 5 is a schematic diagram for a feature point of the surfacemodification method for the light metal casting according to the presentembodiment;

FIG. 6 is a schematic diagram for the feature point of the surfacemodification method for the light metal casting according to the presentembodiment;

FIG. 7 is a schematic diagram for the feature point of the surfacemodification method for the light metal casting according to the presentembodiment;

FIG. 8 is a flowchart showing a procedure of the surface modificationmethod for the light metal casting according to the present embodiment;

FIG. 9 is a schematic diagram for explaining an example of feeding arotating tool and a probe; and

FIG. 10 is for explaining another example of feeding the rotating tooland the probe.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present disclosure is described with an embodiment, butthe claimed invention is not limited to the following embodiment. Inaddition, all the configurations described in the embodiment are notessential to means for solving problems. The following description andthe drawings are appropriately omitted or simplified to clarify theexplanation. In the drawings, the same reference sign is assigned to thesame element, and redundant description is omitted as appropriate.

A surface modification method for a light metal casting according to thepresent embodiment is a method for modifying a surface of a light metalcasting with Friction Stir Processing in which a rotating shaft and arotator are rotated and fed while the rotating shaft and the rotator arebeing pressed against a surface of a casting to modify the surface ofthe casting. The light metal casting is a casting formed of light metal,such as an aluminium alloy or a magnesium alloy. The surfacemodification of the light metal casting is mainly to refine the metalstructure on the surface of the light metal casting and to eliminateinternal defects, such as cavities. First, a friction stir apparatusused in the surface modification method for the light metal castingaccording to the present embodiment is described.

FIG. 1 schematically shows a configuration of a friction stir apparatus1 used in the surface modification method for the light metal castingaccording to the present embodiment. As shown in FIG. 1, the frictionstir apparatus 1 includes a spindle driving part 2, a movement mechanism3, a rotating tool 4 as a rotating shaft, and a probe 5 as a rotator.

The spindle driving part 2 includes a rotating drive shaft (not shown)to be rotated by an electric motor (not shown). The spindle driving part2 is supported by a support column 7 fixed perpendicularly on a pedestalpart 6 through the movement mechanism 3. On the pedestal part 6, aworkpiece table 8 is arranged. On the workpiece table 8, a workpiece Wwhich is a member formed of a light metal casting is placed.

The movement mechanism 3 includes a lifting/pressing mechanism (notshown) and a feeding mechanism (not shown). The lifting/pressingmechanism moves the spindle driving part 2 in the directionperpendicular to the workpiece W (that is, the direction of the rotatingdrive shaft of the spindle). The feeding mechanism moves the spindledriving part 2 in the feeding direction parallel to the workpiece W.

The rotating tool 4 is a cylindrical member to be rotated by the spindledriving part 2, and the central axis of the cylinder is aligned with therotating drive shaft of the spindle driving part 2. At the lower end ofthe rotating tool 4, a pressing surface that is a horizontal facecapable of pressing the upper surface of the workpiece W is formed. Therotating tool 4 is formed of metal material, such as stainless steel,having the higher hardness and a higher melting point than those of thebase material of the workpiece W.

The probe 5 is a cylindrical member having a smaller diameter than thatof the rotating tool 4 and is fixed on the pressing surface of therotating tool 4 so as to project downward from the center of thepressing surface. The central axis of the cylindrical probe 5 and thecentral axis of the cylindrical rotating tool 4 are coaxial. The probe 5is formed of, similarly to the rotating tool 4, metal material havingthe higher hardness and a higher melting point than those of the basematerial of the workpiece W. Note that, the side surface of thecylindrical probe 5 may be threaded in the direction opposite to therotating direction of the probe 5. In this case, when, for example, therotating direction of the probe 5 is clockwise, the side surface of thecylindrical probe 5 is threaded counterclockwise. This promotes stirringa portion of the workpiece W softened by frictional heat.

Next, the surface modification method for the light metal castingaccording to the present embodiment is described. Note that, FIG. 1 isreferred to, as needed, for the configuration of the friction stirapparatus 1 in the following description.

FIG. 2 is a photomicrograph of the metal structure on the surface of amember formed of an aluminium alloy casting (ADT10-F) beforemodification. The black portion in FIG. 2 is eutectic silicon. As shownin FIG. 2, a member formed of an aluminium alloy casting has portions inwhich eutectic silicon is not refined and exists as a coarseprecipitated grain R1 in the metal structure.

FIG. 3 is a schematic diagram for explaining the outline of FrictionStir Processing of the friction stir apparatus 1. As shown in FIG. 3,the lifting/pressing mechanism of the movement mechanism 3 (see FIG. 1)lowers the rotating tool 4 and the probe 5 in the direction of the arrowA, which is the rotating axis direction of the spindle, and the rotatingtool 4 and the probe 5 are rotated at a high speed in the direction ofthe arrow C (clockwise) while the upper surface of the workpiece W ispressed with the pressing surface at the lower end of the rotating tool4. Specifically, the rotating tool 4 and the probe 5 are brought intofrictional contact with the base material of the workpiece W. Thefrictional heat generated at this time maintains the temperature rangein which the light metal material, which is the base material of theworkpiece W, is not melted but plastically deformed, and the basematerial of the workpiece W is softened and stirred.

Since the coarse precipitated grains (see FIG. 2) in the metal structureon the surface of the workpiece W are thereby crushed, the metalstructure is refined, and internal defects, such as cavities, areeliminated.

As described above, the rotating tool 4 and the probe 5 are pressedagainst the workpiece W and rotated at a high speed, and the rotatingtool 4 is moved by the feeding mechanism of the movement mechanism 3 inthe direction of the arrow B, which is the feeding direction. As theresult, the surface of the workpiece W is modified by the frictionstirring along a moving path L of the rotating tool 4. In other words, asurface Wa of the workpiece W subjected to friction stirring ismodified.

FIG. 4 is a schematic diagram for explaining the difference in themodification effect of friction stirring. Here, FIG. 4 is a diagramviewed from the direction of the arrow A in FIG. 3. In FIG. 4, the sideat which the rotating direction C (C1) of the rotating tool 4 and theprobe 5 coincides with the feeding direction B is referred to as anAdvancing Side (AS), and the side at which the rotating direction C (C2)of the rotating tool 4 and the probe 5 and the feeding direction B areopposite is referred to as a Retreating Side (RS). The inventor hasfound that, on the surface Wa subjected to the friction stirring, themetal structure on the surface in the vicinity of the side AS is refinedas compared to in the vicinity of the side RS and that the effect ofimproving the strength and ductility is increased.

FIGS. 5 to 7 are schematic diagrams for explaining a feature point ofthe surface modification method for the light metal casting according tothe present embodiment. Here, FIG. 5 shows a workpiece W 1, which is amember formed of a light metal casting, viewed from the feedingdirection of the rotating tool 4 and the probe 5. FIG. 6 shows, in theupper part, a workpiece W1 viewed from the feeding direction of therotating tool 4 and the probe 5, and FIG. 6 further shows, in the lowerpart, the workpiece W1 viewed from the axial direction of the rotatingtool 4 and the probe 5. FIG. 7 shows the workpiece W1 viewed from thefeeding direction of the rotating tool 4 and the probe 5.

As shown in FIG. 5, the workpiece W 1, which is the member formed of thelight metal casting, is assumed to have a welded portion S1 at whichstress tends to concentrate. It is highly possible that a region S2 inthe vicinity of the welded portion S1 includes internal defects and hasthe lowest strength. Thus, when the workpiece W1 receives an impact, thepossibility of occurrence of a fracture at the region S2 in the vicinityof the welded portion S is high. However, it is difficult, in terms ofdesign, to control occurrence of a fracture at the region S2 in thevicinity of the welded portion S1. For this reason, when the workpieceW1 receives an impact, it is undesirable that a fracture occurs at leastat the region S2 in the vicinity of the welded portion S1. Thus, thestrength at the region S2 in the vicinity of the welded portion S1 needsto be increased to be higher than the strength at regions other than theregion S2, such as a region S3 and a region S4.

For this reason, in the surface modification method for the light metalcasting according to the present embodiment, as shown in FIG. 6, theregion S2 in the vicinity of the welded portion S1 is to be positionedat the side AS at which the rotating direction of the rotating tool 4and the probe 5 coincides with the feeding direction. In other words, aportion at which increase in the strength is desired with modificationof the workpiece W1 is to be positioned at the side AS at which therotating direction of the rotating tool 4 and the probe 5 coincides withthe feeding direction. At this time, the side RS at which the rotatingdirection of the rotating tool 4 and the probe 5 and the feedingdirection are opposite is positioned at the region S3. The rotating tool4 and the probe 5 are positioned in this manner and fed while therotating tool 4 and the probe 5 are rotated.

FIG. 7 further shows photomicrographs of the metal structure on thesurface of the workpiece W1 after the surface modification method forthe light metal casting described with reference to FIGS. 5 and 6 isperformed. Here, the workpiece W1 is an aluminium alloy casting(ADT10-F). In FIG. 7, P1 is a photomicrograph of the metal structure atthe region S2 in the vicinity of the welded portion S1 on the surface Waof the workpiece W1 subjected to the friction stirring. In addition, P2is a photomicrograph of the metal structure at the region S3 on thesurface Wa of the workpiece W1 subjected to the friction stirring, andP3 is a photomicrograph of the metal structure at the region S4. Blackportions in P1, P2, and P3 in FIG. 7 are eutectic silicon.

In FIG. 7, eutectic silicon is finely distributed in P1 that is aphotomicrograph of the metal structure at the region S2 as compared toin P2 that is a photomicrograph of the metal structure at the region S3and in P3 that is a photomicrograph of the metal structure at the regionS4. The region S3 has been positioned at the side RS when the frictionstirring is performed, and has many portions at which the metalstructure has been refined as compared to the base material of theworkpiece W1 shown in FIG. 2 but has not been refined sufficiently. Themetal structure at the region S4 between the region S2 and the region S3has been refined as compared to at the region S3, but has not beenrefined as compared to at the region S2.

As described above, by performing the surface modification method forthe light metal casting according to the present embodiment, it ispossible to further refine the surface at the region S2, which is aportion at which the strength on the surface of the workpiece W1 isespecially required, as compared to at regions other than the region S2,such as the region S3 and the region S4, and to increase the strength.

FIG. 8 is a flowchart showing a procedure of the surface modificationmethod for the light metal casting according to the present embodiment.As shown in FIG. 8, first, the rotating tool 4 and the probe 5 arepositioned with respect to a workpiece so that the side AS at which therotating direction of the rotating tool 4 and the probe 5 coincides withthe feeding direction is positioned at a portion at which increase inthe strength is desired with modification of the light metal casting(step S1). Then, the rotating tool 4 and the probe 5 are pressed againstthe surface of the light metal casting while being rotated (step S2).Then, the rotating tool 4 and the probe 5 are fed along a predeterminedpath (step S3).

FIG. 9 is a schematic diagram for explaining another example of feedingthe rotating tool 4 and the probe 5 (a different example from FIG. 6).Here, the upper part of FIG. 9 is a diagram viewed from the feedingdirection of the rotating tool 4 and the probe 5, and the lower part ofFIG. 9 is a diagram viewed from the axial direction of the rotating tool4 and the probe 5. As shown in FIG. 9, the rotating tool 4 and the probe5 may be fed in the same feeding direction B a plurality of times, andeach feeding may be performed by shifting, in parallel, a next feedingpath L2 from a previous feeding path L1 by a predetermined width J equalto or less than the diameter D of the probe 5. By performing feeding inthis manner, a portion which has not been positioned at the side AS inthe previous feeding path L1 is to be positioned at the side AS in thenext feeding path L2 or subsequent feeding paths, and it is possible toevenly refine the surface of the workpiece W.

FIG. 10 is a schematic diagram for explaining another example of feedingthe rotating tool 4 and the probe 5 (a different example from FIGS. 6and 9). Here, FIG. 10 is a diagram viewed from the axial direction ofthe rotating tool 4 and the probe 5. As shown in FIG. 10, the side AS atwhich the rotating direction of the rotating tool 4 and the probe 5coincides with the feeding direction may be positioned at an edge M of aworkpiece W2, which is a member formed of a light metal casting, and therotating tool 4 and the probe 5 may be fed according to a path L3circling along the edge M. By positioning the side AS at the edge M ofthe workpiece W2 and positioning the side RS at the inner side, themetal structure on the surface at a portion in the vicinity of the edgeM is refined. Thus, it is possible to increase the strength at the outerperiphery of the workpiece W2.

The present disclosure has been described above with the aboveembodiment, but is not limited to the configurations in the embodimentand includes various modifications, amendments, and combinations thatcan be understood by those skilled in the art without departing thescope of the claimed invention.

From the disclosure thus described, it will be obvious that theembodiments of the disclosure may be varied in many ways. Suchvariations are not to be regarded as a departure from the spirit andscope of the disclosure, and all such modifications as would be obviousto one skilled in the art are intended for inclusion within the scope ofthe following claims.

What is claimed is:
 1. A surface modification method for a light metalcasting with Friction Stir Processing in which a rotating shaft and arotator are rotated and fed while the rotating shaft and the rotator arebeing pressed against a surface of a casting to modify the surface ofthe casting, the method comprising feeding the rotating shaft and therotator while rotating the rotating shaft and the rotator in a mannersuch that a side at which a rotating direction of the rotating shaft andthe rotator coincides with a feeding direction is positioned at aportion at which increase in the strength is desired with modificationof the light metal casting.
 2. The surface modification method for thelight metal casting according to claim 1, further comprising: performingthe feeding in the same feeding direction a plurality of times; andperforming each feeding by shifting, in parallel, a next feeding pathfrom a previous feeding path by a predetermined width equal to or lessthan a diameter of the rotator.
 3. The surface modification method forthe light metal casting according to claim 1, further comprising feedingthe rotating shaft and the rotator according to a path circling along anedge of the light metal casting in a manner such that the side at whichthe rotating direction of the rotating shaft and the rotator coincideswith the feeding direction is positioned at the edge of the light metalcasting.